It is sometimes desirable to provide a patient with a high flow (30 liters or more per minute) of pure oxygen mixed with room air at any of a number of constant oxygen concentrations ranging from 24 to 55 percent. Because of the importance of maintaining a predetermined concentration for a patient undergoing treatment, and because a mask must not be encumbered with any bulky or heavy attachments (such as gas mixing devices of the type used for incubators and the like), especially attachments which might be thrown out of adjustment by patient movement or activity, it has been a common practice in the past to provide separate masks for each different oxygen concentration. Ordinarily, four such masks have been made available to therapists for supplying patients with oxygen concentrations of 24, 28, 35, and 40 percent.
Such a pre-set oxygen diluter is disclosed in U.S. Pat. No. 3,714,944. The diluter includes a bulbous mixing chamber with lateral apertures for the entry of room air; when a different concentration of oxygen is required, the unit is disassembled and the enlarged mixing chamber is replaced by another similar chamber having a different arrangement of apertures. U.S. Pat. No. 3,794,072 discloses a single device capable of providing different oxygen dilutions depending on the number of air-restriction closure disks fitted upon the body of the device. Three such disks are shown, resulting in a diluter consisting of four separable parts. Other U.S. Pat. Nos. such as 2,980,106, 2,820,477, and 3,836,079 disclose mixing valves for use with incubators, humidifiers, and the like.